It is lacking the traditional ferrule or compression ring seen in all other types of compression fittings, and it is certainly not a flare or brazed connection. I believe it is a pressed fitting as the name implies. Anyone have any thoughts here?

Al, I would guess he's looking at Dental Air/Vac and if pro press could be used under solder or compression as allowed joining methods. (Guessing here)

According to Viega's approval documents they consider it a compression fitting. The copper is compressed around the gasket(O-ring). A very similar principal to a compression nut and ferrule, but an o-ring in stead of the feerule and the pressure from the press tool instead of tightening the nut.